Biological fluidized bed reactors are commonly employed in waste treatment plants to treat wastewater or any other contaminated matter. The contaminated matter, called the influent, is typically distributed through an inlet conduit and into a tank or vessel of the biological fluidized bed reactor. For example, a flow distributor may be coupled to the inlet conduit and positioned at the bottom of the reactor vessel to deliver the influent to the vessel. Such a flow distributor typically includes a plurality of nozzles configured to inject the influent into the reactor vessel.
The reactor vessel often contains media particles, such as sand, activated charcoal or synthetic beads. The media particles are covered with a biologically active material capable of consuming the contaminants of the influent through enzymatic reaction. More specifically, the biologically active material reduces the biochemical oxygen demand of the influent. It is the activity of the biologically active material (or “biomass”) within the biological reactor that degrades contaminants in the influent to effect a filtration process. The treated matter, called the effluent, buoys above the strata of media particles to the top of the vessel. The effluent is removed from the reactor vessel by mechanical means for further treatment or disposal. The principles of operation of a biological fluidized bed reactor are disclosed in U.S. Pat. No. 3,846,289, which is incorporated herein by reference in its entirety.
Under normal operating conditions of the fluidized bed reactor, a pump or other delivery apparatus typically distributes the influent through the inlet conduit and into the bottom portion of the reactor vessel. The influent flows upwardly through the media particles at a velocity sufficient to buoy the media particles. The flow rate of the influent through the nozzles is sufficient to fluidize the media particles, i.e., induce fluid-like movement of the media particles and suspend the fluidized media particles throughout the interior of the vessel.
In the event of a power loss or interruption, the pump or other delivery apparatus ceases to deliver influent into the reactor vessel. Consequent to the influent flow interruption, the influent and media particles within the fluidized bed reactor may backflow through the nozzles of the flow distributor and into the inlet conduit. Examples of variables that can influence the backflow in the reactor vessel in the event of a power interruption include, but are not limited to, the weight and volume of media particles, position of the inlet conduit relative to the reactor vessel fluid level, elevation of the influent source relative to the elevation of the reactor vessel and existence of gas pockets in the inlet conduit.
Regardless of the origin or cause of the backflow condition, the backflow of media particles can obstruct and clog the nozzles of the flow distributor. Upon reactivation of the pump or other delivery apparatus, the motive influent circumvents the obstructed nozzles and surges through the remaining unobstructed nozzles at a higher velocity. The undesirable arrangement of obstructed and unobstructed nozzles causes a non-uniform flow distribution throughout the reactor vessel. Non-uniform flow distribution attributes to a host of problems, including but not limited to, nozzle abrasion resulting from the high velocity influent, defluidized media particle clusters adjacent the obstructed nozzles, increased influent filtration cycle time and more.
The entire flow distributor is typically removed from the reactor vessel or disassembled within the reactor vessel for repair or replacement to eliminate the media particle obstruction from the nozzles. Servicing the flow distributor is a costly operation accounting for reactor downtime, parts and labor. Therefore, it would be beneficial to provide a system configured to limit or prevent backflow in a biological fluidized bed reactor.
Attempts have been made to incorporate a backflow prevention system into a biological fluidized bed reactor. For example, a backflow prevention system for a media bed reactor disclosed by Mazewski et al. in U.S. Pat. No. 5,766,491, which incorporated herein by reference in its entirety, is illustrated in FIGS. 1 and 2. In this example the fluidized bed reactor of Mazewski et al. comprises a pump 36 that delivers influent through a fluid flow conduit and into a flow distributor positioned at the bottom end of a reactor vessel. A backflow prevention apparatus indirectly coupled to the fluid flow conduit is configured to deliver an auxiliary fluid to the flow distributor in the event influent flow to the flow distributor is interrupted. The backflow prevention apparatus illustrated in FIG. 1, which includes an auxiliary tank 92, auxiliary pump 82 and rechargeable battery 84, is coupled to the fluid flow conduit via an auxiliary fluid flow conduit. By maintaining the auxiliary fluid flow to the flow distributor following an influent flow interruption, media bed constituents are limited from backing up into the flow distributor while the media bed settles.
The battery operated auxiliary pump 82 is configured to deliver the auxiliary fluid from the auxiliary tank 92 through the auxiliary fluid flow conduit. The auxiliary fluid flow conduit delivers the auxiliary fluid into the reactor vessel through the flow distributor nozzles. The backflow prevention system additionally includes a shutdown interlock assembly 94 which comprises a solenoid actuated fail-open valve 68 in the auxiliary fluid flow conduit and a solenoid actuated fail-close valve 70 in the primary fluid flow conduit. The fail-open valve 68 and the fail-close valve 70 are electrically interlocked and operate in response to a signal generated by a power loss indicator 72 that detects power loss to a motor used to drive the primary pump 36.
Nevertheless, there continues to be a need to further develop and improve backflow limiting systems for biological fluidized bed reactors.